This invention relates to current-limiting fuses containing a conductive polymer exhibiting a sharp increase in electrical resistance at a threshold current. The polymer is coupled in series with mechanical breaker contacts and is resistively heated to the threshold temperature in the event of a fault current, to limit current as the breaker opens. A commutation shunt resistance can be coupled in parallel with the polymer. The invention relates to conductive polymer current-limiting fuses which exhibit extremely low let-through values. More particularly, the conductive polymer current-limiting fuses are based on metal-filled elastomer material and exhibit let-through values less than 5,000 A2s with a switch current of 1.79 kAp, preferably less than 2,500 A2s, most preferably no more than 2,250 A2s. Several embodiments are disclosed, including an arrangement is which at least one electrode is in free surface contact with a planar polymer element and urged against the polymer element under a force.
For protecting circuits and loads as well as persons and property, electrical energy advantageously must be controlled nearly instantaneously to react to a sensed fault condition. For example, it is advisable promptly to disengage a load in the event of a short circuit or similar fault condition by cutting off the current supply. If the fault current is not quickly cut off, the potential for serious damage is increased. For this reason, short-circuit protection devices are routinely incorporated in power supply circuits for controllably interrupting a fault current. A circuit protective device may sense and respond to long term current overload, or to a ground fault or a short circuit. In the event of a short circuit. the protective device should operate as quickly as possible.
Devices for controlling current involve one or more means along a current supply conductor, in series with the load, which insert an insulating gap or large resistance when triggered. These include mechanically separable electrical contacts, fuses, thermistors with positive temperature coefficients and others. Elastomers for use in such devices are comprised of all polymers that exhibit elastic properties which are similar to those exhibited by natural rubber. Elastomers can be compressed or stretched within a relatively large permitted elastic area, and return to their original state when the load is removed. Electrically conductive elastomers are a class of rubber and plastics which have been made electrically conductive, either by the addition of metal mixtures or by orienting metal fibers under the influence of electric fields, or by the addition of different carbon mixtures or ceramics, for instance V2O3-material dispersed in the manner described in the article xe2x80x9cV2O3 Composite Thermistorsxe2x80x9d by D. Loffat, et al, published in Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, 1986, pages 673-676. In rubber, there is used several types of xe2x80x9ccarbon blackxe2x80x9d, for instance graphite, acetylene black, lampblack and furnace black with particle diameters ranging from 10-300 nm. Examples of appropriate rubber materials which become electrically conductive after adding metal mixtures or carbon mixtures are butyl, natural, polychloroprene, neoprene, EPDM, and the most important silicone rubber. Additives of metals and metal alloys in powder form suited as elastomer additives are silver, nickel, copper, silver-plated copper, silver-plated nickel, and silver-plated aluminum.
The most common types of carbon or metal-filled plastics are polyethylene and polypropylene. These are used at present for heating cables and for overload protectors, for instance the earlier mentioned polymer-based PTC-thermnistors. However, an electrically conductive filler impairs the mechanical properties of the plastic. The material becomes brittle and hard and is therewith not readily deformed. These materials are therefore less useful as pressure transducers with pressure contacts. A further limitation of carbon-filled plastics resides in their relatively high resistivity, typically one 1 Ohm cm and higher, which limits applications to low power. On the other hand, metal-filled plastics can be produced with significantly lower resistivity, lower than 0.5 Ohm cm, although voltage or tension stability becomes very poor, and consequently these materials are not suited as overload protectors.
Electrically conductive elastomers can be given very low resistances, for instance resistances of 2 mOhm cm or lower, by admixing metal powder. One advantage afforded by elastomers is that they are very soft in comparison with carbon-filled polyethylene and polypropylene, even when containing large quantities of electrically conductive filler. Such elastomers will have a typical Shore number of between 20-80, according American Standard ASTM D2240 (Q/C).
To function as a current limiting element includes at least one electrically conductive elastomeric body and two electrodes. The polymer composition of the elastomeric body may be of various kinds, examples of suitable elastomers including butyl, natural, polychlorpropylene, neoprene, EPDM and silicone rubber. The electroconductive powder material is preferably comprised of silver, nickel, cobalt, silver-plated copper, silver-plated nickel, silver-plated aluminum, lampblack, conductive soot or carbon black. The powder material will suitably have a particle size of 0.01-10 micro-meters and the powder filler is suitably present in an amount corresponding to 40-90% of the combined weight of the powder filler and elastomeric material. The resistivity of the electric elastomeric body will preferably lie within the range of 0.1 mOhm cm-10 Ohm cm. When the device includes more than one electrically conductive elastomeric body, the bodies may be made of mutually the same or mutually different elastomers and then with mutually the same or mutually different fillers and resistivity. The electrodes are of a conventional kind, for instance silver-plated copper. The electrodes are preferably oriented so that repulsion forces will occur between the electrodes when high currents pass therethrough. The pressure achieved on the electrodes, for instance with a known pressure device described in U.S. Pat. No. 3,914,727, or by a conventional spring mechanism for the on/off function of an electric switch, deforms the convex abutment surface of the elastomeric body, when the device includes such an abutment surface. This deformation will preferably reach at least 5%. A deformation of 5-30% is particularly preferred, as defined with a starting point from the distance between the bodies that borders on a considered elastomeric body, i.e. if the distance when the pressure is 0 and bordering bodies lie in abutment with the elastomeric body is d and if the distance changes to 0.7.multidot.d after the pressure has been applied, the body will have been deformed by 30%. Particularly preferred elastomeric bodies are those which have a hardness between 30-50 IRHD in accord with British Standard BS903/A26, although materials having both a lower and a higher hardness may conceivably be used.
Mechanical circuit breakers have the capacity to interrupt high currents.
However, the break-time of conventional circuit breakers is several milliseconds, with even the fastest circuit breakers taking 3 to 5 ms to open and interrupt fault currents. Due to various mechanical constraints and the continuation of conduction as an are is struck and extinguished between the separating contacts, traditional circuit-breaker technology leaves little potential for further reductions in break-times.
The present invention provides a compact, low cost, high power fuse, based on conductive polymers designed to be connected in series with a conventional mechanical circuit breaker. The combination is believed suitable for use at Type II protection levels, i.e., no damage to contactor contacts in a motor starter combination at high fault ratings. When combined with a conventional mechanical circuit breaker, the devices of the invention exhibit extremely low let-through values, preferably less than 5,000 A2s. more preferably less than 2,500 A2s, most preferably no more than 2,250 A2s, with a typical switch current of 1.79 kAp, wherein the switch current is the current at which the device of the present invention rapidly transitions to a high resistance state.
The invention provides a high power fuse capable of increasing the current limiting capacity of mechanical circuit breakers when connected in electrical series therewith, by inserting a series resistance upon the occurrence of a fault current at which the mechanical circuit breaker will throw.
According to an aspect of the invention, a fuse is provided with a metal filled elastomer, two electrodes, a spring, enclosure, and a commutation shunt device. The metal filled elastomer is held in contact with a pair of electrodes through the application of a force sufficient to deform the elastomer at the interface with the electrodes, resulting in a low device resistance.
According to another aspect of the invention, a fuse is provided whose resistance increases at a threshold current, at which point a dielectric barrier forms between the metal particles in the elastomer and the electrodes, increasing sharply the resistance of the fuse, whereupon the current is commutated into a shunt impedance in parallel with the elastomer.
The fuse of the invention may be combined with a conventional circuit breaker for use at Type II protection levels, that is. at voltages up to 480 V and current up to 100 kA. The fuses exhibit fast sharp and well controlled switching characteristics.
According to one embodiment, the fuse comprises a body of an electrically conductive elastomer, wherein the body defines two parallel surfaces. At least two electrodes bear against the body for carrying current. At least one of these electrodes is in free contact with one of the parallel surfaces and a pressure mechanism maintains a force perpendicular to the parallel surfaces to mechanically force the electrodes against the parallel surfaces. A shunt resistor is provided in parallel with the electrodes and the body.
In accordance with the present invention, fuses are provided which comprise a body of an elastomeric composition rendered electrically conductive by a conductive filler. The composition comprises an elastomer selected from silicone rubber, Kraton(copyright) thermoplastic rubber compounds, thermoplastic elastomer (TPE) compounds, ethylene-propylene-diene monomer (EPDM), gum rubbers and the like; and a plurality of conductive particles selected from silver, nickel, aluminum, iron, copper, carbon black and mixtures thereof.